Type XI was more than a U-boat. Much more. It was in fact a U-cruiser. With a submerged displacement of 4,650 tons, six torpedo tubes and a gun armament of four 128-mm guns in two twin gun turrets it would have been the heaviest-armed U-boat and the second-largest (after the Japanese I-400 series). Like the latter, it would have carried a collapsible floatplane – Arado Ar 231 (for long-range recon missions).
The idea behind the development of Type XI was understandable – to remedy the key deficiency of Type VII (the standard U-boat of Kriegsmarine) – its weak gun armament that made it (when surfaced) an easy prey for convoy escort ships (corvettes and the like).
A “wolfpack” of several Type XI U-cruisers had the ability to destroy not only the merchant ships, but escort vessels as well thus annihilating entire convoys (something that the “wolfpack” of Type VII subs could not do).
There was a major problem with this idea, however. The problem that could be expressed in just two words – escort carrier. Escort aircraft carrier that is. Although Type XI boats were supposed to carry formidable anti-aircraft armament as well (two 37-mm and two 20-mm autocannons), it was no match for a squadron of 25-30 anti-submarine aircraft carried by just one such vessel.
Consequently, had these impressive submarines been constructed and put to sea, they would have been inevitably sunk by escort carrier-based aircraft. And rather sooner than later. And the resources used to construct these boats would have been completely wasted.
Fortunately, only four Type XI U-boats (U-112, U-113, U-114, and U-115) were laid down in 1939, and (thankfully) cancelled at the outbreak of World War II. And Kriegsmarine would have been much better off had they concentrated on the next-generation Type XXI subs as soon as possible – these could have been a real game changer due to its high speed, advanced sonar, snorkel and other key features.